Method and system for controlling streamers

ABSTRACT

A method and system for controlling the shape and separation of an arrangement of streamers towed behind a survey vessel. Each streamer is steered laterally by lateral steering devices positioned along its length at specific nodes. Each streamer is driven by its lateral steering devices to achieve a specified separation from a neighboring streamer. One of these actual streamers, used as a reference by the other actual streamers, is steered to achieve a specified separation from an imaginary, or ghost, streamer virtually towed with the actual streamers.

CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.12/613,617, “Method and System for Controlling Streamers,” filed Nov. 6,2009, now U.S. Pat. No. 8,223,585, which claims priority to U.S.Provisional Patent Application Ser. No. 61/112,429, filed Nov. 7, 2008.Both prior applications are incorporated by reference.

BACKGROUND

The invention relates generally to offshore marine seismic prospectingand, more particularly, to systems and methods for controlling thespread of towed seismic streamers.

In the search for hydrocarbon deposits beneath the ocean floor, a surveyvessel 10, as shown in FIG. 1, tows one or more seismic sources (notshown) and one or more streamer cables S₁-S₄ instrumented withhydrophones and other sensors. In multiple-streamer systems, thestreamers are towed underwater behind the survey vessel in a generallyparallel arrangement. The tail end of each streamer is tethered to atail buoy 11 that marks its position. The seismic source periodicallyemits a seismic wave that propagates into the ocean floor and reflectsoff geologic formations. The reflected seismic waves are received by thehydrophones in the streamers. The hydrophone data is collected and laterprocessed to produce a map of the earth's crust in the survey area.

The quality of the survey depends on, among other things, knowledge ofthe precise position of each hydrophone. Position sensors, such asheading sensors and acoustic ranging devices 12, located along thelengths of the streamers are used to determine the shapes of thestreamers and their relative separations. The acoustic ranging devicesare typically acoustic transceivers operating on a range of channelsover which they transmit and receive acoustic ranging signals to andfrom one another to produce accurate ranges 14 between their locationson the streamers. The many ranges—only a few are shown in FIG. 1—andstreamer heading data from the many heading sensors are used to computea network solution that defines the shapes of the individual streamersand their relative positions. When one point on the streamer array istied to a geodetic reference, such as provided by a GPS receiver, theabsolute position of each hydrophone can be determined.

Positioning devices, such as depth-keeping birds and lateral steeringdevices 16 located at nodes along the lengths of the streamers, are usedto control the depths of the streamers and their separations from eachother. The positioning devices could be equipped with acoustic rangingdevices to range with other positioning devices so equipped and withdedicated acoustic ranging devices. Precise positioning of the streamersis important during online survey passes to produce a high-quality map.Cross currents, however, cause the streamers to deviate from straightlines parallel to the towing vessel's course. Instead, the streamers mayangle straight from their tow points or assume a curved shape with theirtail ends tailing away from the straight lines. This feathering of thestreamers is often undesirable in online survey passes. Precisepositioning is also important during turns between online survey runs toreduce the time of the turn without entangling the streamers.

In conventional streamer positioning systems, one of the streamers,outermost port streamer S₁ in this example, is used as a referencestreamer. A shipboard controller 18 collects the position sensor dataand computes the network solution representing the shapes of thestreamers and their separations from each other. From the networksolution and the target separations of corresponding steering-devicenodes on the streamers referenced directly or indirectly to points onthe reference streamer, the shipboard controller derives steeringcommands for each lateral steering device. The steering commands aretransmitted to the steering devices to adjust their control planes, orfins, to drive the streamers laterally, as indicated by arrows 20, tomaintain the target separations.

In current systems, a human operator steers the reference streamer bysending lateral steering commands to the lateral position controllers todrive the reference streamer to adjust feather. The other streamers arethen automatically steered toward the selected separations referenceddirectly or indirectly from the reference streamer. But the manualpositioning of the reference streamer is time-consuming and, in turns,can be hectic as well.

SUMMARY

This shortcoming and others are addressed by a system embodying featuresof the invention for controlling the spread of a plurality of streamerstowed behind a survey vessel. The system comprises a plurality of actualstreamers towed by a survey vessel and having head ends laterally offsetfrom each other. Each actual streamer has a plurality of lateralsteering devices and position sensors disposed along its length. Acontroller receives position data from the position sensors anddetermines the shapes and positions of the actual streamers from theposition data. The controller calculates the shape and position of animaginary streamer towed by the survey vessel and further calculateslateral separations for each of the actual streamers referenced to theshape and position of the imaginary streamer. The controller sendslateral steering commands to the lateral steering devices correspondingto the calculated lateral separations.

In another aspect of the invention, a method for controlling the spreadof N actual streamers S₁-S_(N) towed in the water behind a survey vesseland equipped with lateral steering devices at spaced apart locationsalong the lengths of the actual streamers comprises:

(a) defining the shape and position of an imaginary streamer G_(r); (b)determining the shapes and positions of actual streamers S₁-S_(N); (c)sending lateral steering commands to lateral steering devices on actualstreamer S_(r) to adjust the separation in the water of actual referencestreamer S_(r) from imaginary streamer G_(r); and (d) sending lateralsteering commands to lateral steering devices on actual streamer S_(n)to adjust the separation in the water of actual reference streamer S_(n)from another actual streamer S_(i), for every nε{1, 2, . . . , N} andn≠r and wherein iε{1, 2, . . . , N}, i≠n, and, in at least one case,i=r.

BRIEF DESCRIPTION OF THE DRAWINGS

These features and aspects of the invention, as well as its advantages,are better understood by referring to the following description,appended claims, and accompanying drawings, in which:

FIG. 1 is a top plan view of a survey vessel towing a steamer networkillustrating conventional acoustic cross-bracing and lateral steering ofstreamers using a reference streamer;

FIG. 2 is a top plan view as in FIG. 1 illustrating the use of a virtualstreamer to which a reference streamer is referred, according to theinvention;

FIG. 3 is a block diagram of the streamer positioning control systemusable with the streamer network of FIG. 2; and

FIGS. 4A-4C are flowcharts of a control sequence for the control systemof FIG. 3.

DETAILED DESCRIPTION

The operation of a streamer steering system according to the inventionis described with reference to an exemplary four-streamer system shownin FIG. 2. (The streamer arrangement is the same as that in FIG. 1.) Asurvey vessel 10 tows four steamers S₁-S₄ (generally, S₁-S_(N) for anN-streamer system) whose tail ends 22 are tethered to tail buoys 11.Head ends 23 of the streamers are attached to a system of tow cables andtethers 24 attached to the rear deck of the vessel. Paravanes 26 areused to maintain a wide spread for the deployed streamer network.Lateral steering devices 16—disposed at spaced apart locations, orsteering nodes, e.g., every 300 m, along the length of eachstreamer—exert lateral forces 20 to drive the streamer to starboard orport. A shipboard controller 28 connected to the position sensors andthe lateral steering devices and the streamers by a communications link,such as a hardwired link 30 running along the tow cables and through thestreamers, receives positioning and other data from the position sensorsand transmits steering commands to the lateral steering devices over thelink.

Just as for the streamer system of FIG. 1, the system of FIG. 2designates one streamer—outermost port streamer S₁ in this example—as areference streamer S_(r). The closest neighboring streamer S₂ is steeredtoward a selected separation D from the reference streamer S₁. As shownin FIG. 2, the node for head-end lateral steering device 16A on streamerS₂ is farther away from the distance of closest approach to referencestreamer S₁ than the selected separation. Consequently, the shipboardcontroller issues a steering command to the head-end steering device 16Ato drive the streamer to port, as indicated by arrow 32. Because thetail-end steering node for steering device 16B on streamer S₂ is closerto the distance of closest approach to reference streamer S₁ than theselected separation, as shown in FIG. 2, the shipboard controller issuesthe steering command to the tail-end lateral steering device 16B on S₂to force the streamer to starboard, as indicated by arrow 33. Thesteering commands contain, for example, control values, such as finangle values related to the separation error calculated from the desiredtarget separation and the actual separation between the node on thestreamer at which the lateral steering device is located and the nearestpoint on a streamer to which it is referenced. The lengths of the arrows32, 33 are proportional to the magnitudes of the changes in the finangle settings, which control the azimuth of the control surfaces ofeach lateral steering device. Because streamers S₁ and S₂ are at thedesired separation D about midway along their lengths in FIG. 2, the finangle setting for lateral steering device 16C does not have to change.

Just as streamer S₂ is steered directly referenced to the shape andposition of reference streamer S₁, S₂'s starboard neighbor S₃ is steeredreferenced to the position and shape of S₂. And outermost starboardstreamer S₄ is positioned relative to S₃. This is a preferred mode ofoperation because the ranges between closely spaced, adjacent streamersas derived from the acoustic ranging devices are typically more accuratethan those between farther apart, non-adjacent streamers. In thisexample, each of the streamers S₂-S₄ is steered to maintain a desiredseparation from its port neighbor. Reference streamer S₁ thus serves asa direct reference for streamer S₂ and as an indirect reference for theother streamers.

In one conventional version of a streamer steering system, the referencestreamer S₁ is steered by a human operator giving manual commands viathe shipboard controller to adjust the feather of the reference, whichcauses the streamer to assume a corresponding shape and position.According to the invention, the positioning of the reference streamer isautomated by referencing it to a pre-defined imaginary streamer G_(r).This imaginary, or virtual, or ghost streamer is defined through theshipboard controller according to selectable criteria, such as targetfeather. Once the criteria are selected, the shipboard controllerautomatically drives the reference streamer S_(r) to assume apre-selected separation, which could be zero, from the ghost streamerG_(r). (In FIG. 2, r=1.) The other streamers S₂-S₄ are steeredconventionally as already described. As shown in FIG. 2, the ghoststreamer G₁ is selected with its head end 23′ coincident with the towpoint 34 of reference streamer S₁. In this example, G₁ is also shownwith a certain amount of feather F. (But G₁ could alternatively beselected to be deployed straight behind the vessel or offset from thetow point of S₁.)

Generally, for a system of N actual streamers S₁-S_(N), one of thestreamers is designated a reference streamer S_(r). In a preferredarrangement, r=1 or N to designate an outermost port or starboardstreamer, S₁ or S_(N), for streamers consecutively positioned from portto starboard S₁-S_(N). The associated ghost streamer is then G₁ orG_(N). If the reference ghost streamer G_(r)=G₁, then actual streamer S₁is referenced to G₁, and S_(n), is referenced to S_(n-1) for every nε{2,3, . . . , N}. More generally, if the reference streamer S_(r) isreferenced to ghost streamer G_(r), then actual streamer S_(n), will bereferenced to another actual streamer S₁ for every nε{1, 2, . . . N} andn≠r, and wherein iε{1, 2, . . . , N}, i≠n, and, in at least one case,i=r.

A block diagram of the streamer steering control system is shown in FIG.3. The shipboard controller, which may comprise one or more processors,executes a number of individual processes to control the shape andseparations of the streamers. A control user interface 36 allows anoperator to set various lateral steering parameters. The parameters mayinclude:

-   -   a) Ghost Streamer ON/OFF—enables or disables steering of the        reference streamer to the ghost streamer;    -   b) Reference Streamer—selects which streamer to use as the        reference streamer S_(r) during automatic separation control of        the streamers;    -   c) Max Fin Angle for S_(r)—maximum value of the fin angle to be        used on the reference streamer during online operation (to limit        bias in the fin angle control range and to limit flow noise due        to lateral displacement of the streamer);    -   d) Mode—sets the online operating mode (fan mode or even spacing        mode for streamer shape and separation); and    -   e) Feather—manual setting of the desired feather for a 3D survey        line.

Some of these parameters are sent to a network calculator means 38 thatcalculates the current positions of all the actual streamers S₁-S_(N).(As used in this description and in the claims, “position” meansposition and shape of the streamer, except when explicitly used with“shape.”) The network calculator then calculates the separations betweenthe actual streamers and their corresponding target streamers.

During a typical survey, a realtime planner 40 defines the track thevessel is to follow. Each track consists of a series of turns followedby online passes. The planner defines online shot points along theonline portion of the track and at the ends of the turns and offlinevessel positions during the remainder of the turns. In a 3D survey, thefinal target feather value coming out of a turn is the value manuallyentered via the control user interface. In a 4D survey, target featherangles are set and adjusted along the survey line based on featherangles recorded during the baseline survey the new survey is designed toreplicate.

The realtime planner sends the computed track setting, which includesthe association of feather matching, to an auto-steering means 42, whichcalculates the required future positions of the ghost streamers. Theauto-steering means is a process that computes the range and bearingfrom each positioning-sensor node, as well as from the nodes for otherdevices, on each streamer to the distance of closest approach to thatstreamer's target streamer, whether another actual streamer or avirtual, or ghost, streamer associated with the actual streamer. Theauto-steering process also computes the shape and position of the ghoststreamer G₁, for example, to which the reference streamer S₁ isreferenced. The auto-steering process can also optionally compute theghost streamers G₂-G_(N) associated with each of the other actualstreamers S₂-S_(N) from the calculated actual streamer positionsreceived from the network calculator. (See FIG. 2 for an example of oneof the other ghost streamers G₄, shown with no feather for the purposesof illustration.) This allows the ghost and reference streamers to bechanged from the outermost port streamer to the outermost starboardstreamer, for example. The auto-steering process updates the ghoststreamer's shapes and positions at a generally regular interval in turnsand offline and, typically, once per shot online.

A lateral steering interface 43 receives the streamer separation valuesand, under the control of the control user interface, passes them alongto a lateral controller 44 that converts the streamer separation valuesfor each node into streamer steering commands, including, for example,fin angle commands, and transmits them over the communications link 30to the lateral steering devices 16, which appropriately adjust their finangles.

Sometimes during an online pass, strong currents or navigation orinstrumentation problems can cause the streamer spread to inadequatelycover the survey area. The survey area is divided into a gridwork ofbins. If an insufficient amount of data is collected in some of thebins, the pass will have to be re-run, at least in part, to fill inthose bins with more data. The process of re-running parts of onlinepasses in subsequent online passes to complete the data set is known asinfill. A realtime binning supervisor process 46 monitors the binsduring a line and adjusts the spread of the streamers to minimize theamount of infill needed. The required track derived form the binningdata to meet the infill requirements is sent to the auto-steeringprocess, which then calculates a corresponding ghost streamer for thepass.

Another process is used to supervise the spread of the streamer systemduring deployment by helping to automate the distribution of streamerseparations as the streamers are payed out from the back deck of thesurvey vessel. A deployment supervisor 48 sends track and deploymentdata to the auto-steering process, which calculates the ghost streamerposition for the network calculator to control the lateral steeringdevices.

A flowchart of the streamer steering process that runs once each shotpoint online or at a regular interval offline and in turns is shown inFIGS. 4A-4C. First, the shipboard controller computes the networksolution 50 from ranges gathered from the acoustic ranging devices andheadings from the heading sensors. From the network solution, the actualstreamer positions are updated 52. These steps are performed by thenetwork calculator. Next, the auto-steering process updates the ghoststreamer position from the desired feather 54. The network calculatorthen calculates the ghost-to-reference-streamer separation 56 (forexample, between each node on S₁ and its closest point of approach toG₁) and the separations from each of the other actual streamers to aneighboring streamer 58 (between nodes on S_(n) and their closest pointsof approach to S_(n-1)). Once the separations at each node in thenetwork are calculated, the lateral steering interface, as controlled bythe control user interface sends 60 the separations to the lateralcontroller.

The lateral controller calculates 62 lateral separation-error termscorresponding to the separations of a lateral steering device from oneor more actual or ghost streamers. 62. Closed-loop controls, such asproportional-integral (PI) control loops, calculate 64 new fin anglesfor the devices' control surfaces, or fins. A motor in each steeringdevice drives the fins to the new fin angle 66. While the lateralsteering devices are being controlled to steer the streamers, theshipboard controller triggers a new data acquisition cycle 68 duringwhich the position sensors (heading sensors and acoustic rangingdevices) acquire streamer position data (headings and acoustic ranges)70. The position sensors then send 72 the position data over thecommunications link to the controller for the network calculator tocompute the network solution during the next positioning data updatecycle, typically once per shot online and at some regular rate whenrunning offline or in turns between lines.

Although the invention has been described with reference to a preferredversion, other versions are possible. For example, the PI control loopsthat run in the shipboard lateral controller could be performed, forexample, individually in each of the steering devices on the streamers.In that case, the steering devices would receive the necessaryseparation values from the shipboard controller in a command message. Asyet another example, the system described is adaptable to a multi-vesselsurvey, in which most of the shipboard controller functions areperformed by a master controller aboard one of the vessels linked toslave controllers aboard the other vessels. The slave controllers wouldbe devoted largely to interfacing with the positioning-sensors andlateral steering devices. The master controller would perform most ofthe other functions, such as computing the complete network solution,defining the track for each streamer, and defining the ghost streamerfor each vessel's streamer network. A radio or other wirelesscommunication link would allow the master controller to communicate withthe slaves on the other vessels. As another example, the block diagramdefines a number of discrete blocks performing specific functions. Thenames of these blocks and the functions they perform were arbitrarilyassigned to simplify the description of the system. The variousprocesses may be distributed across blocks in many ways to similareffect. So, as these few examples suggest, the scope of the claims isnot meant to be limited to the preferred version described in detail.

What is claimed is:
 1. A system for controlling the spread of aplurality of streamers towed behind a survey vessel, comprising: aplurality of actual streamers towed by a survey vessel and having headends laterally offset from each other, each actual streamer having aplurality of lateral steering devices and position sensors disposedalong its length; a controller receiving position data from the positionsensors and determining the shapes and positions of the actual streamersfrom the position data, calculating the shape and position of animaginary streamer towed by the survey vessel, calculating lateralseparations between one of the actual streamers, nominated a referenceactual streamer, and the imaginary streamer, calculating lateralseparations for each of the other actual streamers referenced to theshape and position of the reference actual streamer, and sending lateralsteering commands corresponding to the calculated lateral separations tothe lateral steering devices on the actual streamers.
 2. The system ofclaim 1 wherein the controller calculates lateral separations betweeneach of the other of the actual streamers and the reference actualstreamer.
 3. The system of claim 1 wherein the controller calculateslateral separations between each of the actual streamers and an adjacentactual streamer.
 4. A method for controlling the spread of N actualstreamers S₁-S_(N) towed in the water behind a survey vessel andequipped with lateral steering devices at spaced apart locations alongthe lengths of the actual streamers, comprising: defining the shape andposition of an imaginary streamer G_(r); determining the shapes andpositions of actual streamers S₁-S_(N); sending lateral steeringcommands to lateral steering devices on actual reference streamer S_(r)to adjust the separation in the water of actual reference streamer S_(r)from imaginary streamer G_(r); sending lateral steering commands fromthe controller to lateral steering devices on actual streamer S_(n) toadjust the separation in the water of actual streamer S_(n) from anotheractual streamer S_(i), for every nε{1, 2, . . . , N} and n≠r and whereiniε{1, 2, . . . , N}, i≠n, and, in at least one case, i=r.
 5. The methodof claim 4 wherein i=n−1.
 6. The method of claim 5 wherein actualstreamers S₁ and S_(N) are outermost actual streamers on opposite sidesof the spread and actual streamers S₂-S_(N-1) are sequentially offsetlaterally between the outermost actual streamers.
 7. The method of claim4 further comprising defining the shapes and positions of N imaginarystreamers G₁-G_(N) corresponding to actual streamers S₁-S_(N).
 8. Themethod of claim 4 wherein the shape and position of the imaginarystreamer G_(r) is determined from the required track and from featherdata.
 9. The method of claim 4 wherein the shape and position of theimaginary streamer G_(r) is determined and the separations of the actualstreamers S₁-S_(N) are adjusted to control the spread of the streamersduring deployment and retrieval.
 10. The method of claim 4 wherein theshape and position of the imaginary streamer G_(r) is determined and theseparations of the actual streamers S₁-S_(N) are adjusted to minimizethe need for later infill.